Role of PM H+-ATPase in citrate exudation by cluster roots of P-deficient lupin

White lupin (Lupinus albus L.) exhibits a high efficiency in acquiring P from sparingly soluble soil sources through the release of huge amounts of P-mobilizing root exudates such as citrate and protons into the rhizosphere. The present work aimed at assessing the involvement of the plasma membrane (PM) H+-ATPase in the release of citrate from cluster roots of P-deficient white lupin plants. Citrate exudation from cluster roots followed a diurnal rhythm with a maximum 5 h after the beginning of the light period; at this time a massive efflux of protons into the root external medium could also be monitored. The release of citrate observed during the first 5 h following the onset of the light period was accompanied by an increase in the cytosolic pH. The pattern of citrate release was paralleled by changes in the activity and the amount of the PM H+-ATPase. Fusicoccin, which stimulates PM H+-ATPase activity, promoted release of citrate and protons from cluster roots even during the phase of the light period characterized by low citrate efflux. Results suggest that PM H+-ATPase is strongly involved in the process of citrate release by cluster roots of P-deficient white lupin plants